A hybrid or electric vehicle uses electrical energy, which is stored in a battery, as an energy source. In order to operate such a battery, it is crucial in this case to be able to determine the state of the battery and the capacity of the battery for continued operation. For this purpose, the operation of the battery is managed using a battery management system. Battery packs using Li ions or NiMH technology, which consist of a large number of electrochemical cells connected in series and/or in parallel, are thus used in hybrid and electric vehicles. The battery management system is used to monitor the battery and, in addition to monitoring safety, is intended to ensure a service life which is as long as possible.
For battery management, the voltage of each individual cell is measured, together with the battery current and the battery temperature, and the state of charge and the state of ageing, for example, are estimated. In order to maximize the service life, it is necessary to know the instantaneous maximum power capacity of the battery at any time. If this power limit is exceeded, the ageing of the battery may be rapidly accelerated.
Methods which are active during operation are known for determining the battery state, in particular the state of charge (SOC). These methods are based on the relationship between the battery voltage and the SOC. However, the open-circuit voltage (OCV) is relevant here. Since the methods are active during operation, the measured voltage must be corrected by the voltage drop caused by the flowing current and the internal resistance of the battery. For this purpose, use is made of impedance models which contain errors. DE 100 980 A1 thus discloses a battery management system for determining the state of charge of the battery from the measurement of the internal resistance Ri. The only time at which the correct OCV can be measured is during starting of the system following a pause, that is to say during initialization.